The goal of this research is to understand the molecular mechanisms of spontaneous and genotoxicant induced mutagenesis of repeated DNA sequences that can adopt alternative secondary structures. The applicant continues to test the hypothesis that certain DNA sequences form secondary structure substrates for spontaneous mutagenesis resulting in DNA-directed mutation events (DNA-directed mutations). Mutations associated with these DNA sequences cause cancer and human disease. The first aim is to understand the molecular mechanisms responsible for triplet repeat instability associated with thirteen neurodegenerative diseases, including myotonicdystrophy type 1, Fragile X syndrome, and Friedreich's Ataxia, using sensitive genetic assays for repeat instability in Escherichia coli. A second aim involves understanding the molecular mechanisms and leading/lagging specificity of duplication mutations. A third aim seeks to understand the basis for a mutation hotspot for intermolecular strand switch events that correct quasipalindromes to perfect inverted repeats. The fourth aim will determine the role of DNA replication, repair and recombination in spontaneous DNA secondary structure mutagenesis. The fifth aim will test the hypothesis that certain chemical and environmental mutagens increase the rate of DNA secondary structure mutagenesis. Dr. Sinden will examine the effect of numerous chemical and environmental genotoxicants, including UV irradiation, X-radiation, mitomycin C, ethylnitrosourea (ENU) and ethylmethane sulfonate (EMS), cisplatin, N-methyl-N'-nitro N-nitrosoguanidine (MNNG), polycyclic aromatic hydrocarbons, and N-nitrosamines including 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and N'-nitrosonornicotine (NNN). These experiments will provide an understanding of the molecular mechanisms and pathways involved in spontaneous and genotoxicant-induced mutagenesis.